The present invention relates to an exposure method and an apparatus therefor which is suitable for enlarging and transferring a pattern formed on a mask onto a substrate during the process of manufacturing electronic devices such as a semiconductor, a liquid crystal display element, and the like.
The liquid crystal display element, for example, is thought to be a promising display because of its configuration, which is thinner and smaller than that of an electron tube. A liquid crystal display element of active matrix type which employs thin film transistors (TFT) has been becoming widely used because of its excellent picture quality.
For manufacturing the TFT, it is necessary to use an exposure device performing in substantially the same manner as an exposure device for manufacture of a semiconductor element. Consequently, a proximity method type exposure device and a one-to-one mirror-and-lens-projection method type exposure device are used.
There is the prospect that the size of the display will become as large as the screen of a CRT (cathode-ray tube). If such a large display is actually produced, however, there will arise various problems with the use of existing exposure devices.
The proximity type exposure method has problems when exposing a large area, such as occur in the manufacturing of a large area and high precision mask, high precision detection of a gap between mask and substrate, and reduction of pitch error.
On the other hand, the projection type exposure method, inevitably causes a joint portion in the screen due to its exposing style. This method makes it difficult to attain a high precision and satisfactory electrical characteristic values in the joint portion. Further, since separate exposures are carried out, the throughput is low and it is difficult to reduce the production cost of the device due to its exposing style.
In order to solve the above-described problems of the existing exposure methods, a method can be considered wherein a mask produced by an electronic drawing machine used for manufacturing high precision masks, for example, a semiconductor mask not larger than seven inches, is enlarged through a projection optical system so as to achieve an exposure over a large area. An example of such an enlargement projection type exposure method is disclosed in Japanese Patent Unexamined Publication No. 62-122126, in which a pattern formed on a mask is enlarged through a projection optical system onto a substrate, the projection optical system employing parallel rays on the substrate side thereof.
The optical system used with the enlargement projection type exposure method mentioned above serves to project a pattern as parallel rays onto the substrate side thereof. In an ordinary reduction projection type exposure device used for semiconductors, parallel rays are employed on the projected side or on the wafer side, with the result that there is hardly any shape error with respect to the deviation in the focal direction.
However, it is inevitable that the lens used in the projection optical system will differ from the ideal values even if it is manufactured to high accuracy. Errors include pattern distortion, magnification error and the like. The absolute values of these errors become large in the case of enlargement projection.
For example, even with a lens having a pattern distortion factor of 0.01%, a deformation of 50 .mu.m appears in the periphery on a screen with each side being 500 mm. It is still necessary, therefore, to achieve a smaller value than the above value by one figure because of the accuracy of the superposition among the layers forming the patterns. Further, every lens has its own individual characteristics and it is impossible to manufacture lenses that are identical with one another, so that it is impossible to achieve a superposition of patterns by making use of a plurality of devices. Even if the pattern distortion and the magnification error come out in accordance with each individual lens, it is considerable that, when there is no fluctuation or a very small fluctuation, a single device is used to form patterns throughout the entire process. However, since the relation between the device and the product is limited, there gives rise to a problem that the capability of mass-production is reduced. In addition, in case of achieving an exposure over a still larger area by means of joining patterns together, there is a problem in that superposition is not assured at a joint portion due to deformation.